Hydraulic brake systems in high performance or race vehicles utilize either dual master cylinders or a production-style tandem master cylinder. Tandem master cylinders use one cylinder bore with two pressure ports and pistons. They are designed so that if pressure is lost in either port, the other port maintains its pressure. Dual master cylinder set-ups completely isolate the two hydraulic systems. One system connects to the front wheels, while the other connects to the rear wheels. Whichever method is utilized, dual brake systems will still provide braking force from one system even if the other one completely loses pressure.
There are several options available when choosing a performance brake system, including, among other things, different brake pad compounds, rotors of various sizes and weights, calipers with different piston sizes, and the ability to change the bore size of a master cylinder.
With so many variables involved with proper brake tuning, race teams have begun to realize that many spring, shock, sway bar, and chassis weight adjustments can be eliminated with a small adjustment to the brake system.
Changes brought to a car when brakes are applied include deceleration drag on the driveline from the engine, weight transfer from rear to front and side to side, and stress on tires. It is imperative, given those variables, that a brake system be properly balanced, in order to minimize the shock that is delivered to a chassis. If not, corner handling will be lost and the driver's comfort and concentration will be compromised.
Improperly tuned brakes can create handling problems. Heat, in particular, can cause problems. On tracks where braking is critical, heat can boost front tire pressures and decrease handling performance, leading to slower lap times and inconsistent performance.
An important objective when tuning the chassis with brakes is having the right amount of front-to-rear brake bias in the car. Brake bias is defined as the total braking force distributed between the front and rear tires expressed as a percentage. A 70–30 bias split from front to rear is a general guideline for oval track racing on asphalt. Dirt track racing, road racing and varying track conditions on any type of track, can lead to different split requirements. The range can vary from as high as 75–25 down to 58–42, with 70–30 bias split being a baseline figure.
Many factors affect the amount of braking force a tire can generate. The most important one is the force (weight, downforce, etc.) pushing the tire against the ground. As a car decelerates, weight is transferred from the rear to the front tires. This weight transfer reduces the amount of braking force the rear tires can produce. Application of too much brake force to the rear wheels will cause lock-up, causing the rear end to lose traction and possibly swing around violently.
When entering a corner, 60 to 80% of the weight is transferred to the front of the car. The exact amount of weight shift depends on the speed of the car, track, corner, and how much brake is applied upon entry. Because of these varying factors, more or less front brake needs to be “dialed” into the car.
As a means for controlling the amount of brake force which is applied at the front and rear wheels of the car when the brakes are applied, a balance bar may be utilized. A balance bar is positioned between the front and rear master cylinders. The position of the bar can be adjusted to allow more or less brake pressure to be applied towards the front when the brake pedal is pushed.
In general, the balance bar is an adjustable lever (usually a threaded rod), that pivots on a spherical bearing and connects to the two separate master cylinders for the front and rear brakes. Most balance bars are part of a pedal assembly that also provides a mounting for the master cylinders. When the balance bar is centered, it pushes equally on both master cylinders creating equal pressure, given that the master cylinders are the same size bore. When adjusted as far as possible toward one master cylinder, it will push approximately twice as hard on that cylinder as the other.
The driver can adjust the balance bar before, after or during a race, or practice sessions, to change the precise adjustment of the front to rear bias setting, allowing for changing track conditions or to optimize brake performance. The correct front brake bias setting depends on the driver's driving style and how hard or how much the brakes are used getting into a corner. Since this will vary with each corner at each track, it is important to find the right balance as not to upset the chassis when the brakes are applied while cornering.
Unfortunately, the balance bar is only a simple mechanism for comparatively changing the amount of brake force which is applied to the front and rear wheels. The balance bar does not ensure that the proper amount of braking force is applied.
Besides the above described factors affecting brake force, the configurations of the various brake system components affect brake force. For example, it is well known that mechanical parts have manufacturing tolerances whereby “identical” parts may be physically larger or smaller but still within an acceptable range. Brake calipers, brake pads, brake rotors, brake mounting components and related parts all experience this issue. In addition, the piston seals used in brake calipers offer different resistance levels to piston movement in the caliper due to the manufactured piston and bore tolerances. It is desirable, for maximum performance, to have all four brake calipers operating in a consistent manner with the same characteristics.
In an effort to find “balanced” sets of calipers that have the same performance characteristics, many auto race teams and brake caliper manufacturers measure the hydraulic pressure required to operate the caliper. Calipers that fall within an acceptable range are designated as matched sets and are used on race vehicles to enhance that vehicle's braking performance. Other brake system components can also contribute to an unbalanced system, including master cylinder and mechanical pedal variations along with other mechanical mounting variations of system components.
As one attempt to determine braking force, some prior art methods utilize hydraulic line gauges. These gauges measure brake line pressure. This manner of attempting to determine brake force suffers from several disadvantages. First, the gauges are not highly accurate and repeatable. Further, the hydraulic line pressure gauges only measure the brake line pressure to establish or record vehicle brake bias settings. Because brake line pressure is only a measurement of the force applied behind the caliper pistons, the line pressure has to be multiplied by the total area of the caliper pistons as one factor in determining clamping force on the rotor. This is a difficult task. Further, as indicated above, other factors may affect the actual braking or clamping force.
Various objects, features, and advantages of the present invention over the prior art will become apparent from the detailed description of the drawings which follows, when considered with the attached figures.